Has anybody installed GT40 heads on their 351w carbureted engine? I was wondering if I would have to mill down the head bolts? Any input will be greatly appreciated!

A lot of folks have done the GT-40 upgrade, you don't need to "mill" down the head bolts, the head bolt holes on the gt-40 heads need to be opened up to 1/2" because the 351w uses 1/2" head bolts and the typical gt-40 302 cylinder head uses 7/16" head bolts.

__________________
/spôrk/
a spoon-shaped eating utensil with short tines at the tip

Do the gt40 heads need different valves or cam to operate correctly? How about the exhaust?

All existing parts should work. Just drill out the head bolt holes, bolt the cylinder heads on, check lifter preload if within specs put valve covers on and call it a day and enjoy your new found 20-30 hp. If you want the heads to operate more smoothly you should replace the intake manifold and if you want more hp replace the camshaft although replacing the camshaft can open a can of worms. Lots of threads on here already about different guys using different parts to achieve more hp and torque...

__________________
/spôrk/
a spoon-shaped eating utensil with short tines at the tip

Not all GT40 heads came on 302's... so you may or may not need to have the bolt holes drilled out. GT40p heads only came on 302's and would require drilling. Those heads are cheaper, more readily available and generally make more power for this application, so you may want to look into those instead.

As mentioned above, valvetrain parts will indeed swap over from stock heads, but do get a shim kit in order to dial in lifter preload. If you cant get by with .090" of shims (or less) then you may want to consider new (shorter) pushrods.

Agreed that a cam and intake upgrade at the same time would give you the best bang for your buck. Dont forget to install a new double roller timing set as well.